This invention relates generally to shear type shredders and, more particularly, to automatically reversible hydraulic drive arrangements for such shredders.
Hydraulically driven, shear-type shredders are disclosed in U.S. Pat. No. 3,868,062 to Cunningham, et al. and U.S. Pat. No. 4,034,918 to Culbertson, et al. Prior to the drive arrangements disclosed in those patents, a shear-type shredder was typically driven by an electric motor through a high speed reduction gear train. Any jamming condition occurring in the shredder was transmitted directly to the motor through the gear train. The motor was provided with electric current-sensing and motor-reversing circuitry to detect a jamming condition in the shredder and reverse the electric motor briefly to clear the jam. The arrangement was satisfactory for small shredders having a maximum rating of approximately 20-30 horsepower. By comparison, the high torques required by larger shredders, coupled with frequent jamming and reversing sequences, often caused the electric motors to overheat and burn out.
Accordingly, it was proposed that such shredders be driven hydraulically by interposing a hydraulic pump, motor, and fluid circuit with pressure relief valves between the shredder mechanism and the electric motor. The electric motor would then drive the hydraulic pump. Persons involved in shear-type shredder design believed that this arrangement would effectively isolate the electric motor from excessive torque loads due to jamming conditions in the shredder, and thereby prevent burnout. The earliest hydraulic shredder drive designs employed hydraulic sequencing valves in their hydraulic circuits which both detected jamming conditions upon an increase in hydraulic pressure and briefly actuated a flow-reversing valve in the circuit to reverse the hydraulic motor and thereby clear the jamming condition. This design operated erratically due to both variations in fluid viscosity with temperature and resultant difficulties in determining a consistent reversal pressure threshold.
To correct these problems, as well as others, the aforementioned patents proposed drive arrangements which continued to both sense jamming conditions and actuate a flow reversal means in the hydraulic circuit, but did so with electrical means rather than with hydraulic means. More specifically, those designs employed hydraulic pressure-actuated electric switches, electrically operated pneumatic timers and control relays, and electric reversal solenoids. By electrically sensing overpressures in the hydraulic circuit and electrically reversing the hydraulic shredder motor, the reversing cycle was no longer subject to hydraulic fluid temperature and viscosity variations.
These electric-hydraulic reversing circuits, however, introduced several new problems. One problem was the initiation of unintended reversals when the shredder jammed momentarily on tough or excess material and then cut through the material. Another problem was frequent failure of hydraulic pressure-actuated switches.
Both problems are characterized by momentary pressure spiking in the hydraulic circuit. Due to the relative incompressibility of the fluid, a momentary jamming condition in the shredder causes the pressure in the hydraulic circuit to rise very quickly. When the shredder mechanism breaks through the material being shredded, hydraulic pressure suddenly decreases. This momentary rise and fall in hydraulic pressure forms a pressure spike. Such a momentary jamming condition often causes pressure spikes of sufficient magnitude to actuate the hydraulic pressure switch and thereby initiate a reversing cycle. Even though a true jamming condition had not occurred and reversal subsequently proves unnecessary, the reversing sequence, once initiated, would continue until completion.
Each reversal cycle is about one to three seconds duration. In shredding tough materials, such as truck tires or sheet aluminum, true jamming conditions can occur up to several times a minute but usually occur less often. However, momentary jamming conditions occur more frequently, typically a half dozen or more times a minute. Under these conditions, a significant portion of available shredding time can be lost.
This problem is especially significant in very large, for example, 300-600 horsepower shear-type shredders, not only because of the greater dis-economy of unnecessary reversals, but because such large machines are also more prone to pressure spikes. Small shredder drives use high speed electric or hydraulic motors with reduction gear trains which provide sufficient angular momentum to help cut through tough material and thereby help overcome momentary jamming conditions without initiating unintended reversals. However, very large shredders use high torque, low speed radial piston motors with little or no speed reduction gearing. Hence, they have proportionately less angular momentum to assist in overcoming momentary jamming conditions. Minimal angular momentum is preferred so that the large shredders can reverse quickly without damage to the drive arrangement, but it makes such machines more prone to spiking and, therefore, unnecessary actuation of reversal.
One proposed solution to this problem employs a second timer in the electrical reversing control circuit between the pressure switch and the reversal actuation and timing circuitry. This timer is started when the pressure switch is actuated by either a momentary or a true jamming condition. Upon completion of its timing interval, about one-half second, this timer starts the reversal cycle if the pressure switch is still actuated, indicating a true jamming condition. If the pressure switch is no longer actuated, indicating a momentary jamming condition which has been relieved, the reversal cycle is not started and the shredder continues shredding uninterrupted.
While this approach reduces the amount of unnecessary reversing, it does not prevent overuse of the pressure switches, which causes them to wear out sooner than desired. It has, therefore, been proposed to modify the hydraulic fluid circuit to include fluid accumulators and flow constrictors to filter out pressure spikes due to momentary jamming conditions.
Some improvement in operation was noted, but not enough to enable elimination of the second timer or to prevent premature failure of the pressure switch. In addition, the second timer and added hydraulic components are expensive and unduly increase the complexity of the drive arrangement. It would be preferable to avoid such complexity because of the dirty environment in which such shredders are used and the difficulty of maintaining and adjusting both the hydraulic and electrical control circuits by servicemen without special training. It would also be desirable to avoid relying on failure-prone components, such as pressure-actuated electrical switches.
Accordingly, there remains a need for an improved automatically reversible hydraulic drive arrangement for shear-type shredders.